1. Field of the Invention
The present invention relates to eyeglass lens manufacturing and more particularly to an apparatus for finishing eyeglass lens edges.
2. Related Prior Art
In contrast to the edge of circular eyeglass lenses, the circumference of non-circular eyeglass lenses, which have to be shaped so as to match the eyeglass frame, represents a three-dimensional curve. Every point along the circumference of the non-circular eyeglass lens, this non-circular shape having been imparted by preliminary grinding, exhibits a certain radius from the center point of lens rotation, which differs from the radii of the adjacent points along the circumference. The circumference of the eyeglass lens thus can be defined unequivocally by specifying a radius value and the corresponding angle. Due to the curvature of the eyeglass lens, the grinding point will migrate, i.e. the point of contact between the eyeglass lens and the grinding wheel will wander in a direction parallel to the rotation axis of the eyeglass lens. Providing the circumference of the eyeglass lens is then to be provided with an outwardly projecting ridge or inwardly-projecting groove, it may occur at the transition from sections of the eyeglass lens circumference with longer radii to sections of the eyeglass lens circumference with shorter and from transitions with small curvature that the ridge or the groove runs off course, which will cause difficulties particularly in lenses with positive diopter values having relatively thin edge areas when compared with the center section of the eyeglass lens. These difficulties are particularly serious when the edges of the lens are to have a groove cut, into which a cord is laid for attachment to the eyeglass frame. The grinding wheel diameter being as a rule considerably larger than the diameter of the eyeglass lens aggravates this difficulty, which cannot be eliminated by grinding the groove or ridge in a manner where guidance or control is parallel to the axis of eyeglass lens rotation. This guided or controlled grinding can be effected either purely mechanically or by means of a CNC concept with computer control, whereby the computer also controls the grinding of the circumference in accordance with a pre-determinable eyeglass lens contour. Such CNC controlled eyeglass lens edging machines are known.
In addition, the edges of thick lenses with high negative diopter values, where the ridge is narrower than the edge of the eyeglass lens, become very sharp during the grinding of the circumference, forming a slight bevel is desirable here which, due to the three-dimensional curves described by the forward and rearward faces resulting from the shape of the eyeglass lens, cannot be effected with the normal grinding wheel with its much larger diameter than the eyeglass lens. In addition, there are eyeglass frames in which the eyeglass lenses are held in place by means of claws which engage with notches in the eyeglass lenses. These notches can be cut only on special machines.
A grinding wheel designed to keep the groove from running of course has already been proposed in utility model specification G 88 01 224.7, held by the present applicant, in which the rounded circumference of the grinding wheel, fitted with a grinding ridge, exhibits one or more interruptions. Although an improvement in regard to the groove running off course was achieved with this grinding wheel design, it was nonetheless determined that the groove running of course could not be avoided with certainty for every eyeglass lens.